SysTick_IRQn在ARM芯片中也广泛使用，中间的k是小写的。core_riscv.h文件中提到的SysTick中的k也是...
这个和优先级组设置有关，如果优先级组设置是1（NVIC_PriorityGroup_1），那么这里的抢占优先级就是1了
Systick_IRQn is a negative value, you cannot use NVIC_EnableIRQ() with Systick. The out-of-bounds register access probably causes your hardfault. The systick interrupt is enabled as soon as the corresponding bit in Systick->CTRL is set. However, NVIC_SetPriority() supports Systick_IRQn ...